1. Field of the Invention
This invention relates to a method for bonding a cubic boron nitride sintered compact, and in particular to a novel method for bonding a cubic boron nitride sintered compact to another cubic boron nitride sintered compact or to a body of shank material at temperatures in the neighborhood of 750.degree. C.
2. Description of the Prior Art
A cubic boron nitride sintered compact, hereinafter also referred to as cBN sintered compact, has a wide range of potential applications such as in cutting tools, in structural materials and in heat sink materials because of its good thermal conductivity and superior hardness surpassed only by diamond. A cBN sintered compact not containing any binder material is particularly promising in that the excellent properties on the cBN compact are not impaired by the presence of a binding material.
Various methods have been proposed to produce a binder-free cBN compact. For example, a reactive sintering process is disclosed in Japanese Laying-Open Gazette No. 28,782/1985.
A number of proposals have also been made recently for bonding a cBN compact to metals or metal alloys. One of the most commonly employed method is to bond the cBN compact and other metallic material together using a suitable solder applied at the bonding interface thereof. This method is carried out with solder materials based on Au or Ag such as Au--Ta, Au--Nb, Ag--Ti, and Ag--Cu--Ti are known and used, see, for example, Japanese Laying-Open Gazette No. 134,665/1984. Using a suitable solder material, the bonding of the cBN compact to a body of metal or metal alloy is performed in an inert atmosphere or in a vacuum at elevated temperatures above 800.degree. C. Therefore, for the successful solder bonding of the cBN compact, it is required that the cBN compact remains thermally stable at least up to a temperature of 800.degree. C. On the other hand, in the binder-free cBN compact made by the reactive sintering process, a catalyst such as magnesium boron nitride which has been added to hexagonal boron nitride, inevitably remains in small amounts in the cBN compact as a impurity component. This impurity residue has an adverse effect on the cBN compact by significantly impairing its thermal stability. While a cBN compact made by using ceramics or a metal or metals as the binder, is thermally stable at temperatures around 800.degree.-900.degree. C., the cBN compact produced by the reactive sintering technique exhibits appreciable degradation of its properties such as thermal conductivity and hardness at temperatures over 750.degree. C. because fine cracks or fissures are caused in cBN crystal grain boundaries at the elevated temperatures. It is believed that the fine cracks are produced by the great thermal stress generated within the cBN compact during heating due to a substantial difference in thermal expansion between the cBN and the residual magnesium boron nitride.
Accordingly, if it is desired to solder bond such a reactive sintered cBN compact, a soldering temperature of less than 750.degree. C should be used. Also, when the cBN compact is utilized as the structural material, it is subject to a maximum temperature of between 500.degree.-600.degree. C., making it necessary to employ a soldering material which does not melt out at the specified temperatures. It is, therefore, desired to provide a solder which has a soldering temperatures of 650.degree.-750.degree. C., preferably around 700.degree. C. when a melting point is 640.degree.-740.degree. C., and yet is capable of firmly bonding the reactive sintered cBN compact.
Conventionally, Sn containing solders such as Cu-Sn-Ti have been proposed, see, for example, Japanese Laying-Open Gazette No. 136,605/1986. Because of their relatively lower soldering temperatures, the metal component e.g. Ti in the solder which imparts a bonding capacity to the cBN compact, does not enter into a sufficient reaction with the cBN compact, resulting in a poor bonding strength. Besides, such metallic components in the solder tend to be readily oxidized. In order to prevent the solder component from being oxidized during soldering, special care must be taken. For example, soldering must be conducted in a high vacuum of 10.sup.-3 torr or in an ambient of high purity inert gas, or using a suitable oxide getter.